Here is the latest version of the terrain google map with eBird API D3 overlay! In this incarnation, when the user hovers over a circle, the latitude and longitude from the original d3.json call to the eBird API is fed into a second d3.json call to the Google maps API for elevation data and then the bird hotspot location, the number of birds sighted and the elevation are displayed in a tooltip.
It took me some serious time to figure out how to partition the code so that the elevation data was passed to the global variable in the correct order so that the tooltip code could access that data. StackOverflow helped me learn more about callback functions.
Update: page does not function due to CORS issues, investigating . . . .